In the semiconductor industry, step-and-repeat alignment and exposure systems are employed in the processing of semiconductive wafers to form integrated circuits. Very large scale integrated circuits are often fabricated by utilizing a precisely controlled stage to successively position adjacent regions containing an integral number of individual microcircuits on a semiconductive wafer with respect to an image (formed by a projection lens of the step-and-repeat alignment and exposure system) of a reticle containing a next level of microcircuitry that is then printed on the semiconductive wafer at each of those regions. This step-and repeat printing operation forms an array of adjacent regions of microcircuitry on the semiconductive wafer in rows and columns in an ordered parallel and orthogonal manner. Successive processing of the semiconductive wafer and printing of a further level of microcircuitry, aligned with the preceding processed regions to a high (sub micron) accuracy, are typically employed in the fabrication of integrated circuits from the semiconductive wafer. Successful alignment of the preceding processed regions requires the use of an alignment system that can acquire alignment information from the semiconductive wafer and use it to position the precisely controlled stage so that the semiconductive wafer is properly aligned. Most prior alignment systems feature line edge detecting techniques that are slowed in their operation by mechanical scanning, electronic scanning and/or computation time.
Various alignment systems and an alignment mark system that eliminate these causes of alignment slowing were disclosed in the aforementioned and incorporated U.S. patent application Ser. No. 695,400. These alignment systems are of both light and dark field types. The dark field alignment systems have proven to be more acceptable than the light field alignment systems. The alignment systems are limited in application to step-and-repeat alignment and exposure systems featuring catadioptric unit magnification lenses.
What is needed is a dark field alignment system comprising a modified refractive reduction lens and light source together with a microscope sub-system that enables the use of an alignment mark system such as that shown in U.S. patent application Ser. No. 695,400 in a step-and-repeat alignment and exposure system utilizing a projection lens of a refractive reduction type.
The alignment mark system, that is disclosed in U.S. patent application Ser. No. 695,400, utilizes amplitude information from light reflected from a succession of lines appearing within an illuminated area on the semiconductive wafer to provide an alignment signal. This results in an alignment mark system that features large acquisition range and provides real time amplitude and vectorial alignment information. However, the alignment mark system has been limited in its alignment accuracy because of localized variations in light intensity and diffraction characteristics of the light reflected from the alignment marks previously printed on the semiconductive wafer. These variations have resulted in limited alignment accuracy because of concomitant variations in the amount of light entering a light detector device from otherwise identical line edges.
What is needed is an improved alignment mark system which has the accuracy characteristics of a line edge detection system but maintains the large acquisition range and real time amplitude and vectorial alignment information capabilities of the alignment mark system presented in U.S. patent application Ser. No. 695,400.